In optical communications systems that use wavelength division multiplexing (WDM), multiple wavelengths of light are used to support multiple communications channels on a single fiber. Optical amplifiers, such as erbium-doped fiber amplifiers (EDFAs), Raman amplifiers, and the like, are used in these optical communications systems to amplify optical signals that experience attenuation over multi-kilometer fiber optic links. Due to the length of such multi-kilometer fiber optic links, which may be on the order of hundreds or thousands of kilometers, many of these optical amplifiers may be used.
The intrinsic gain spectrum of an EDFA, for example, is not flat. Unless the signal spectrum is equalized, different wavelengths of light are amplified by different amounts as they propagate through multiple optical amplifiers. This results in degraded bit error rate (BER) performance for some communications channels. This degradation is referred to as the “optical amplifier gain ripple penalty.” It comes from two different sources, namely erbium-doped fiber (EDF)/gain flattening filter (GFF) mismatch and the temperature dependent change in the gain spectrum of the optical amplifiers, and it is compounded by the use of many optical amplifiers, increasing linearly with the number of optical amplifiers used. The gain ripple penalty stems from the optical amplifiers and, to a lesser extent, from dispersion compensating modules (DCMs), the fiber plant, etc., and it is usually a reach-limiting constraint for an optical communications system.
Conventional approaches for alleviating the optical amplifier gain ripple penalty involve the use of a dynamic gain equalizer (DGE) or the like. The DGE has a loss spectrum that tracks the gain spectrum of the optical amplifiers, thereby introducing losses to offset gain peaks. Conventional approaches for alleviating the optical amplifier gain ripple penalty also involve the use of an optical performance monitor (OPM) or the like for the active measurement of communications channel signal gain ripple at a receiver or an intermediate point in the optical amplifier chain and a feedback mechanism to provide this communications channel signal gain ripple information to a controlling element, such as a variable optical attenuator (VOA), DGE, or the like. The drawback to these approaches is that OPM and DGE elements are relatively expensive, especially when used in low channel count optical communications systems.
Thus, what is needed in the art is an approach for alleviating the optical amplifier gain ripple penalty that does not necessarily use OPM and DGE elements, or feedback mechanisms, thereby limiting optical communications system cost.